Case-hardening composition



Patented Mar. 17,1931

UNITED ST ES PATENT OFFICE ERNEST G. MOFFEIYT, OF WOODBRIDGE, NEW JERSEY, ASSIGNOR TO AMERICAN OY- ANAIMID COMPANY OF NEW YORK, N. Y., A CORPORATION OF MAIITE CASE-HABDENING COMPOSITION No Drawing.

This invention relates to the case hardening of metals, more particularly to a composition adapted to constitute or form part of a molten bath in which metals are immersed for the purpose of hardening the surface thereof.

Alkali metal cyanides have for a long time been used in the molten state for the purpose of case hardening ,metals, but the use of alkali metal cyanides is attended by anumber of disadvantages and to avoid this it has been proposed to use calcium cyanamid for case hardening. In accomplishin this it was customary to first provide a'mo ten bath of inorganic salts, such as mixtures of calcium chl ride and sodium chloride, potassium 'chloride and sodium chloride and the like. The calcium cyanamid in the" form of lumps was added ,to the molten salts and the bath used for case hardening. However, a number of diiiiculties were encountered in using a bath of this character in that insoluble materials rapidly collected and the bath itself gradually lost its case hardening properties; Another difliculty lay in the fact that calcium cyanamid is lighter than the mixture of inorganic salts causing it to float on the surface, where the heat and air caused combustion and consequent loss thereof. In order to remedy this it has been proposed to place the calcium cyanamid in lump form in a perforated basket which was immersed in the molten salts. This eliminated the difilculty offloatin of the material but the cal- 5 cium cyanami was quickly coated with a layer of lime formed in the reaction so that the bath became inactive more quickly and the calcium cyanamid was incapable of exerting its case hardening properties.

Attempts have been made to overcome the difiiculties encountered in case hardening baths of this character by reactivating the same by subjecting it to the action of an alkali metal cyanide or to the action of'an easil oxidizable metal, such as aluminum, but suc 1927. Serial No. 209,644.

remedies were only temporary. It was necessary to repeat the purifications at frequent intervals.

Even this did not give any great eificiency in the use of the calcium cyanamid as over per cent of the same was lost and the character of the hardening of the metals left much to be desired. The hardening of the metal surface was neither uniform nor sufficiently great for ordinary commercial practice and the depth of penetration of the case was comparatively little.

I have investigated the effects of various materials in molten baths for case hardening metals and I have discovered that if in place of calcium cyanamid I use an alkali earth metal cyanide, such as calcium cyanide and particularly if I use in conjunction therewith an alkali earth metal salt, such as calcium chloride, I obtain a result which is far supperior to anything which had previously been obtained by theme of calcium cyanamid ina bath containing an inorganic salt or .the use of sodium cyanide in a similar bath.

The alkali earth metal cyanide which I prefer to use is a crude calcium cyanide of the type described and claimed in the patent to Landis, #1,359,257 dated November 16, 1920, for cyanide compound and process of producing the same, in which is described a method of making cyanides of the alkali earth metals in the presence of a flux. Cyanides of this type generally tively large roportion of the alkali earth metal cyanide, generally from 35 per cent to 60 per cent thereof mixed with lesser quantities of the flux, and relatively small amounts of calcium cyanamid, carbon and free lime as im urities.

In t e use of my material for case hardening I provide a bath consisting of equal proportions of calcium chloride and sodium chloride and add thereto a suitable amount of crude calcium cyanide at suitable inter vals. For instance, I have provided a case contain a relaimpurities of the calcium hardening mixture of 90 pounds of equal parts of calcium chloride and sodium chloride and added thereto one pound of calcium cyanide at regular intervals and obtained the results given in the following table:

Sclero- Depth Time Temp. scope of reading case 11:0011:10 Inch 11:15-11:30 57 0.008 2nd nails 11:45-12:00 52 0.006 1 1b. lump calcium cyanide in 12:00-12:05 3rd nails 12:20-12:35 1616F. 52 0.010 4th nails 12:45 1:00 1544 53 0. 008 1 lb. lump calcium cyanide 1:00 1:02 5th nails 1:15 1:30 1544 52 0.009 6th nails 1:45 2:00 1544 0.008 1 lb. lump calcium cyanide. 2:00Al1 in.

at once 7th nails 2:15 2:30 1598 55 0.009 81:11 nails 2:45 3:00 1544 54 0.009 1 lb. lump calcium cyanide- 3:00 9th nails 3:15- 3:30 1544 59 0.008 10th nails 3:45- 4:00 1520 57 0. 009

The calcium cyanide upon being added to the bath is melted almost immediately and is equally distributed throughout the mass.

if desired 1 may add to the bath relatively large blocks or briquets of calcium cyanide in which case the specific gravity thereof is sufiicient to cause the same to float just under the surface of the molten salts and to dissolve gradually therein without loss by decomposition orcombustion. A bath of this character needs practically no purification and it retains its'activit indefinitely provided that calcium cyani e is added at suitable intervals in the ratio in which it is consumed in the case hardenng.

The use of calcium chloride in large quantities in the bath insures a high degree of solubility of residual lime therein. so that no trouble is encountered by the accumulation of large amounts of insoluble lime in the bath. I may increase the dissolving power of the bath for lime by changing the proportions of calcium chloride to sodium chloride so as to render the calcium chloride the major constituent. One such bath which has been used with success consists of two parts calcium chloride and one part sodium chloride. A mixture of this composition has a meltin point of about 510 C. The alkali eart metal cyanide which I use contains some residual calcium cyanamid and free carbon which are very finely divided and distributed uni formly through the mass of cyanide. When such cyanide is melted in the case hardening bath this residual calcium cyanamid being finely divided, is readily dissolved and the reaction in the pot probably results in a combination of the cyanamid with the finely divided carbon to form cyanide which, in turn, is the active case hardeningsubstance. In this manner I utilize not only the case hardening qualities of the cyanide but also the cyanide and obtain aseaeoo practically complete utilization of all of the carbon and nitrogen in the material to accomplish case hardening.

The results obtained by the use of calcium cyanide are markedly superior to the results obtained when sodium cyanide is used in place thereof. The hardness of the resulting material is considerably greater when calcium cyanide is used, the depth of case hardening is considerably. greater and much more uniform in the case of calcium cyanide, and the penetration is much more rapid. The table given below shows a run in which sodium cyanide was used in a bath of similar composition to the one given above, excepting that sodium cyanide replaced calcium cyanide. The bath consisted of 90 pounds of a mixture of equal parts of calcium chloride and sodium chloride to which sodium cyanide was added as shown in the table:

Sclero- Depth Time Temp. scope of reading case 1 lb. sodium cyanide Inch 1st nails 11:45-12:00 1400 F. 37 0. 003 2nd nails 12:45- 1:00 1580 38 1 lb. sodium cyanide 1: 15 Frothed over 3rd nails 1:15- 1:30 1620 54 0.008 4th nails 1:45- 2:00 1535 39 0.002 11b. sodium cyanide 2: 15 5th nails 2:15- 2:30 1535 59 0.008 61:11 nails 2:45? 3:00 1535 52 0. 005 7th nails 3:00- 3:15 1500 51 0. 004 1 1b. sodium cyanide 3: 15 8th nails 3:15- 3:30 1500 52 0. 007' 9th nails 3i45 4:00 1560 49 0. 008

About 1" sediment in pot 10th nails 4:00- 4:15 1580 48 0. 006

The bath boils and froths when sodium cyanide is used.

To shoW the relative merits of the present invention as compared to the old practice of using lump calcium cyanamid in the bath the table below shows the results obtained as to hardness and depth of case hardening by treating steel in a manner comparable to the treatments described above, but using calcium cyanamid in place of the cyanides given above:

3% charge of calcium cyanamid. 10:17 a. m. to

It will be noted that in this instance the hardness of the metal was relatively low and after a-short time the depth of the case became so little that it could not be accurately measured.

Although I have attempted to find adequate explanation for the action of calcium cyanide in place of both sodium cyanide and calcium cyanamid in a case hardening bath, I have so far been unable to find a reasonable explanation of the difference in the action. The unexpected result obtained by the substitution of calcium cyanide for other previously used case hardening materials has been obtained by me time after time, and all the results show an undoubtedly superior action of calcium cyanide.

I have decreased the time required for case hardening from two hours which had been used on certain pieces of work when sodium cyanide was used, to one hour by substituting calcium cyanide therefor. The case formed was .012" compared to .008 for the sodium cyanide, an increase of 50%. The cost of -'materials per pound of metal treated is about half. There is no necessity for subjecting the case hardened material to a draw as the core is tough when the hardening is accomplished with calcium cyanide whereas it is brittle if sodium cyanide is used. The invention has been in commercial use for a considerable time'and the results have confirmed the utility of calcium cyanide.

Although I have described my invention setting forth specifically a bath consisting of a mixture ofseveral salts in which calcium cyanide is dissolved, it is of course obvious that but a single salt or various mixtures in any proportions of more than two salts may 'be used. The results obtained by the practice of my invention as given in the tables are merely illustrative of the action of my invention and it may be possible to obtain greater hardness and greater depth of penetration by changing the relative quantities of materials in the bath, the temperatures and the time'of ca'se hardening. I am not limited to the use of the alkali earth'metal cyanides described in the Landis patent as other cyamdes of similar character are equally well adapted for the practice of my invention, the scope of which is given in the claims appended hereto.

What I claim is:

1. A molten bath for'case hardening metals consisting essentially of a minor proportion of an alkali earth metal cyanide with a mixture of salts including an alkali earthmetal halide and an alkali metal halide, the alkali earth metal halide forming at least 50% of the mixture, the alkali earth metal of the halide andof the cyanidebeing the same.

2. A- molten bath for case hardening metals consisting essentially of a minor proportion of an alkali earth metal cyanide with a mixture of salts including an alkali earth metal halide and an alkali metal halide in the proportion of about two arts alkali earth metal allde to one part a kali metal halide, the

alkali earth metal of the halide and of the of an alkali earth metal cyanide with a mixture of salts including a calcium halide and an alkali metal halide in the proportion of about two parts calcium halide to one part alkali metal halide.

5. A molten bath for case hardening metals consisting essentially of aminor proportion of an alkali earth metal cyanide with amixture of salts including an alkali earth metal halide and a sodium halide, the alkali earth metal halide forming at least 50% of the mixture, the alkali earth metal of the halide and of the cyanide being the same.

6. A molten bath for case hardening metals consisting essentially of a minor proportion of calcium cyanide with a mixture of salts including calcium chloride and sodium chloride in the proportion of about two parts calcium chloride to one part sodium chloride;

In testimony whereo I have hereunto subscribed my name this 28th da of July 1927.

ERNEST C. OFFETT. 

